1. Field of the Invention
The present invention relates to a mother glass composition for gradient-index lens suitable for manufacturing a gradient-index lens that is free from lead and having excellent weather-resistance. The present invention also relates to a gradient-index lens obtained from such a mother glass composition for gradient-index lens and a manufacturing method thereof as well as an optical product and an optical device each including the gradient-index lens.
2. Related Background Art
A gradient-index lens is in the shape of a rod or a fiber, and has a refractive index distribution that varies from its center to its periphery in the section thereof. The gradient-index lens is used extensively, for example, in optical products and optical devices, such as fiberscopes and optical communication devices. The gradient-index lens also is used extensively as an imaging optical element equipped to a copying machine, a facsimile, and an LED printer in the form of a lens array in which these lenses are aligned in arrays.
The gradient-index lens can be manufactured, for example, by an ion exchange method. To be more concrete, according to the ion exchange method, a mother glass composition molded into the shape of a rod or a fiber (glass fiber) is immersed in a molten salt. By exchanging an alkali metal element included in the composition and an alkali metal element included in the molten salt (these elements are of types different from each other) through the counter diffusion effect of ions, a refractive index distribution is provided to the inside of the glass fiber, and a gradient-index lens thus is formed.
Reference 1 (JP51-21594B) discloses a gradient-index lens with small chromatic aberration that is obtained through ion exchange between a glass fiber including Cs and a molten salt of potassium nitrate. Reference 2 (JP59-41934B) discloses a glass composition that includes Li2O and Na2O and is suitable for manufacturing a gradient-index lens having a large angular aperture. Reference 3 (JP7-88234B) discloses a glass composition for gradient-index lens including Li2O and Na2O. From this glass composition, it is possible to manufacture a gradient-index lens having an angular aperture of 13° or larger and an effective field of view area of 90% or larger by regulating the molar ratio of these components to fall within a specific range.
Incidentally, there has been a need for a lead-free lens that is substantially free from lead from an environmental viewpoint in recent years. To this end, the applicant of the present invention discloses mother glass compositions for gradient-index lens that are substantially free from lead, for example, in Reference 4 (JP2001-139341A), Reference 5 (JP2002-121048A), Reference 6 (JP2002-211947A), and Reference 7 (JP2002-284543A).
However, when an attempt is made to manufacture a gradient-index lens having a large angular aperture using a glass composition that is substantially free from lead, properties of the manufactured lens, in particular weather resistance, are deteriorated in some cases. This is attributed to the fact that in order to eliminate the content of lead, which had been a component necessary to increase an angular aperture of the lens, the content of Li that also has an effect of increasing an angular aperture has to be increased, which eases the migration (increases the mobility) of alkali ions in the glass composition. The applicant developed a mother glass composition for gradient-index lens that is substantially free from lead and yet capable of reducing the mobility of alkali ions by adding elements that inhibit migration of alkali ions, such as Ba, Sr, Ca, and Mg, while controlling the content of Li adequately. This mother glass composition is disclosed in Reference 8 (JP2005-298775A).
In the glass composition disclosed in Reference 8, the mobility of alkali ions is reduced, and by using this composition, it is possible to manufacture a gradient-index lens that is excellent in various properties, such as weather resistance, strength, and reproducibility of angular aperture, in comparison with a case where a conventional Li-based lead-free glass composition is used. The weather resistance, however, is not necessarily satisfactory, in particular the exposure to moisture, and it is to be hoped that the weather resistance will be further improved.